Polycrystalline silicon, also known as polysilicon, poly-Si or poly, is widely used to form MOS transistor gate electrodes and form interconnections in MOS circuits. Additionally, doped polysilicon can be used to form a resistive element for an integrated circuit. In particular, a polycide resistor is formed of polycrystalline silicon with a layer of silicide positioned on top of the polycrystalline silicon layer. Often the polycrystalline silicon layer is doped with a dopant material to achieve a desired resistance value. The resistance of doped polysilicon is high and as such, it is commonly known in the art to provide a layer of silicide, such as tungsten silicide on top of the polysilicon resistive layer to reduce the overall resistance of the resistive element.
A voltage is provided across two contact points on the surface of the silicide layer to establish the resistor. The equivalent circuit for such a polycide resistor is two resistors in parallel, one representing the resistance through the silicide layer and the other representing the resistance through the polysilicon layer. It is known that the resistance of the polysilicon layer is much higher than the resistance of the silicide layer, and as such, the majority of the current in the circuit flows through the silicide layer. It has been shown that the conventional polycide resistor offers a positive voltage coefficient in the order of 20-200 ppm/V due to the inter-grain boundary resistance.
Fluctuations in the resistance value of polycide resistors occur in response to changes in temperature and applied voltage. The voltage coefficient of resistance (VCR) of a polycide resistor is a measurement of the change in resistance with applied voltage and is expressed as the rate of change in resistance value per 1 volt in the prescribed voltage range (ppm/V). One of the inherent characteristics of polycide resistors is the positive voltage coefficient of the resistor. The positive voltage coefficient results from the increase in the resistance value of the polycide resistor caused by a voltage drop within a grain of the polycrystalline layer, resulting in disruption of the inter-boundary depletion equilibrium. As such, the resistance value of a polycide resistor increases with an increase in applied voltage.
A temperature coefficient of resistance (TCR) is expressed as the change in resistance of a resistor in parts per million for each degree of change in temperature (ppm/° C.). TCR is typically referenced from +25° C. and changes as the temperature increases or decreases. Polycide resistors are known to exhibit a positive temperature coefficient of resistance, such that their resistance value increases with an increase in temperature.
Resistors are used in all analog and mixed-signal circuits. A wide variety of polysilicon resistors utilizing CMOS technologies are available to accommodate the needs of the circuit designer. Figures of merit for resistors include the voltage coefficients of resistance and the temperature coefficient of resistance for the resistor. In the design of such integrated circuits, it is often important to be able to reduce the variation in the resistance value of the resistors over the operational voltage and temperature range. As such, for integrated circuits employing polysilicon resistors it is desirable to have a means of compensating for the positive voltage coefficient and positive temperature coefficient that are inherent characteristics of polysilicon resistors within an integrated circuit design.